Step 2: Tie the Rubber Bands Together.

Step 3: Making the Ammo Cup

Make a hole large enough for the rubber bands to fit through. (remember to make space for the knots to slip through when tying)
If you are using a thin plastic cup you might want to put tape on the edge of the hole so it doesn't cut the rubber band. (see pic)

Step 4: Finishing Off

Step 5: Tips

Ammunition: Water bombs is an obvious choice, I have a few mandarin trees so i use the dropped fruit. Be creative, experiment with different ammo and find the best.

Firing: The best way to fire it is to get 2 friends to hold each end of the rubber band and pull the cup as far back as you can. Then get the two people holding the end of the rubber band ends to pull them forward just as you let go of the cup. If you don't have any friends (lol) just find a place to tie it so it's like a giant slingshot. If you want to fire the projectile high into the air, get the two people holdig the rubber band to stand on a table or something similar. (see pics)

Problems: If the projectile doesn't release from the cup make sure that the cup isn't bending into an oval shape from the strain of the rubber bands. If this is a problem, when pulling it back, keep the cup a round shape by holding it where the rubber bans are tied to the cup. If this still deosn,t work make sure the rubber band are spaced evenly or try using a different/wider cup. If your cup is too long this may also cause problems. Comment if there are any further problems and i will be happy to help.

Variations: You can easily make this on a smaller/larger scale. You could also try making 2 rows of rubber bands for added strength.